1. Field of the Invention
The present invention relates to a method and a system for reading radio frequency identification (RFID) tags. More particularly, the present invention relates to a method and a system for reading printed RFID tags.
2. Description of the Related Art
FIG. 1 is a schematic diagram showing a conventional RFID reader 101 and three RFID tags 102-104. The reader 101 emits ultra high frequency (UHF) or high frequency (HF) electromagnetic waves to enable the tags 102-104 so that the tags 102-104 can backscatter tag signals to the reader 101. The data sent by an RFID tag is modulated in the tag signal. In order to detect the data, the RFID reader has to receive the tag signal and then generate the data based on the tag signal. This data generation process is known as signal detection, decoding, or demodulation.
For example, the RFID tags 102-104 may use the FM0 modulation scheme to encode the data. FIG. 2 is a schematic diagram showing the four data patterns of the FM0 modulation scheme. There are four patterns S1-S4 in FIG. 2. S1 or S4 stands for a data symbol-1. S2 or S3 stands for a data symbol-0. According to the FM0 modulation scheme, there must be a level transition (from low level to high level or from high level to low level) between two consecutive symbols. FIG. 3 is a schematic diagram showing exemplary tag signal waveforms based on the FM0 modulation scheme, in which the waveform 301 is a perfect noise-free tag signal waveform.
There are two conventional methods for signal detection, namely, edge detection and matched filtering. Edge detection detects the transition of a tag signal waveform across the middle line 305, and then reconstructs the data bits according to the transition statistics of the waveform. For example, a data symbol-1 has no transition inside its symbol period; while a data symbol-0 has a transition at the middle of its symbol period. Edge detection is only practical for tag signal waveforms with relatively low noises. As shown in FIG. 3, the waveform 302 is a noisy tag signal waveform received at a distance of 2 meters, which carries the same data as the waveform 301. Although the waveform 302 is distorted by noises, its data pattern is still recognizable by edge detection. On the other hand, the waveform 303 is another noisy tag signal waveform received at a distance of 8 meters, which also carries the same data as the waveform 301. Severe noises cause many false crossings of the waveform 303 over the middle line 305. As a result, edge detection would misinterpret the data pattern.
Matched filtering is more noise-tolerant than edge detection. Matched filtering reconstructs the data bits by pattern matching between the data patterns S1-S4 and the tag signal waveform. However, for matched filtering to work correctly, the data clock rate of the tag signal must be stable and well known. Stable data clock rate is not always available, especially for printed RFID tags.
Printed RFID tags are literally printed by ink-jet printers. Printed RFID tags are much cheaper than conventional chip-based tags. However, printed RFID tags have the disadvantage of very unstable data clock rate. A 20-50% swing of data clock rate is typical. Furthermore, their data clock rate is always drifting. The data clock rate of a symbol may be different from the data clock rate of the next symbol. Consequently, it is impractical to apply either edge detection or matched filtering on noisy tag signals from printed RFID tags.